1. Field of the Invention
The present invention relates to a process for removing sulfur dioxide (SO2) and nitrogen oxides (NOx) from flue gas using chlorine dioxide (ClO2), and more particularly, to a process for removing sulfur dioxide and nitrogen oxides from flue gas, in which chlorine dioxide is allowed to react with flue gas in an alkaline solution so as to simultaneously remove sulfur dioxide and nitrogen oxides from the flue gas.
2. Description of the Prior Art
Flue gas exhausted from the combustion of fossil fuels, such as petroleum and coal, contains various components, typical pollutants of which include sulfur dioxide and nitrogen oxides.
Such sulfur dioxide and nitrogen dioxides pollute the atmosphere, and rain containing such pollutants becomes so-called “acid rain” which is the main cause of soil and river pollution.
Thus, flue gas from the combustion of fossil fuels should emit pollutants in amounts lower than those specified in regulations. For this reason, many studies on systems and/or processes for removing sulfur dioxide and nitrogen oxides from flue gas are being conducted.
To evaluate effects on the removal of nitrogen oxides (NOx) from flue gas, studies on various inorganic oxidizing agents and inorganic oxidizing agents have been conducted.
Among studied oxidizing agents, additives applicable in a vapor phase include ozone, chlorine dioxide, sulfur phosphorus and oxygen, and additives applicable in a liquid phase include KMnO4, NaClO2, H2O2 and ferrous chelate compounds.
Generally, the additive is added into an absorption scrubber so as to oxidize nitrogen oxide (NO) into nitrogen dioxide (NO2) which is relatively easy to treat. The nitrogen dioxide is removed by absorption into an alkaline solution.
Several technologies relating to processes of using the oxidizing agent in this wet process to oxidize nitrogen oxide (NO) into nitrogen dioxide (NO2) and treating the nitrogen dioxide are known in the art.
Meanwhile, technologies for treating sulfur dioxide (SO2) and nitrogen oxides (NOx) contained in flue gas are widely known, but there is still no economically successful technology.
The main reasons for this are that additives usable for the removal of NOx are chemically unsuitable for the removal of SO2 and that the optimum conditions for the action of one of the additives or absorbing agents for NOx and SO2 are not the optimum conditions for the other, even if there is no substantial interference therebetween.
Such phenomena can be seen in the limited conditions for Fe(II) chelate compounds and sulfites disclosed in several patents.
Up to now, among chemical substances related with the absorption of nitrogen oxide (NO), sodium chlorite was identified to be the most efficient oxidizing agent, but it has a problem in that it shows optimum action only in weakly acidic conditions.
Namely, sodium chlorite shows excellent oxidizing powder in an acidic atmosphere with low pH and has excellent absorbing powder in a basic atmosphere with high pH. Thus, pH becomes a critical factor in absorbing NO2 resulting from the oxidation of NO.
Also, the sodium chlorite is a very expensive chemical substance because it is prepared by the reduction of chlorine dioxide as shown in the following reaction formula (1):2ClO2+H2O2+2NaOH→2NaClO2+O2+H2O  (1)
In view of this point, the direct use of chlorine dioxide gas in oxidizing NO is considered to be more reasonable because it can solve not only the problem of cost but also the problem of pH adjustment. Thus, the present invention comprises introducing chlorine dioxide gas into a scrubbing liquid so as to simultaneously treat NOx and SO2 contained in flue gas.
Chlorine dioxide, used in the present invention to remove sulfur dioxide and nitrogen oxides from flue gas, is a neutral component which is promising as a powerful oxidizing agent and sterilizing/disinfecting agent, and recently, its commercial utility has been acknowledged in a wide range of applications, such as bleaching, oxidation and sterilization/disinfection, as have its environmental benefits.
Although chlorine dioxide is well known as a powerful oxidizing agent capable of oxidizing NO into NO2 in vapor and liquid phases, an effort to use chlorine dioxide to simultaneously treat SO2 and NO contained in flue gas has not yet been made. Thus, there is now an urgent need for the technology for using chlorine dioxide to treat industrial flue gases.
Methods of preparing chloride dioxide have been reported in a wide range of literature, including a plurality of patents. Chlorine dioxide is generally prepared in situ and used immediately, because it has risk factors due to its instability and rapid decomposition.
Chlorine dioxide is prepared from an acidic solution of NaClO2 or NaClO3, and medium and small-scale ClO2 generators mostly utilize NaClO2 as a raw material. When a large amount of chlorine dioxide is required, NaClO3is then used.
Chlorine dioxide can be produced either by the oxidation of chlorite as shown in the following reaction formula (2) or the reduction of chlorate with various organic or inorganic substances in a relatively concentrated acidic solution as shown in the following reaction formula (3):ClO2−→ClO2+e−  (2)ClO3−+2H++e−→ClO2+H2O  (3)
The selection of the reducing agent has a great effect on the optimum reaction condition, by-products and process economy.
The process of preparing ClO2 from NaClO2 is easier to control than the process of preparing ClO2 from NaClO3, but this process is uneconomic.
In fact, the reaction formula (2) is reversible, and chlorite may be synthesized from ClO2. Thus, from an industrial point of view, NaClO3 is considered to the most suitable raw material for preparing ClO2.